Hello NGFeng,
the Trainers Valley requirements are
120 Kanto pokemon caught
+40 evolved
defeating the Red boss npc
You can find the trainstation in Goldenrod City and in Saffron City, you have to pay 5k first and then choose Trainers Valley and pay another 10k
let me know if this helped :)
https://imgur.com/a/vxcmK pretty sure I should have the 120 kanto pokemons